Method of forming molybdenum silicide coating on molybdenum



United States Patent 3,12,065 METHOD 0? FQERMING MGLYEDENUM SILHCIDECGATKNG 0N MGLYBDENUM John P. Page, Thousand Oaks, Roger D. Mueller,Simi,

and George V. Sneeshy, Paeohna, Calif, assignors to North AmericanAviation, Inc.

No Drawing. Filed June 1, 1962, Ser. No. 199,267

8 Claims. (Cl. 117-414) Our invention relates to a method of providingan oxidation-resistant coating on molybdenum, and more particularly toan improved method of providing a molybcnum silicide coating onmolybdenum.

Refractory metals, that is metals with melting points above that ofchromium (34-05 R), are of increasing importance. The advanced newtechnologies require metals capable of withstanding extremely hightemperatures, and frequently material-s problems are key limitations tomore rapid advancement in the air-craft and space fields, The refractorymetals are the subject of intensive research and development because oftheir high temperature strength characteristics. The refractory metalsof principal interest are tungsten, tantalum, molybdenum,

' and columbium.

Molybdenum is unique in its relative availability and low material cost,plus its combination of low density, high melting point, and hightemperature strength. The refractory metals, however, have very pooroxidation re sistance because, unlike many other metals, they do notform their own thin, surface-protective oxide coating. Considerableeffort is bein expended in the evelopment of oxidation-resistantcoatings for refractory metals. See for example the article,Oxidation-Resistant Coating for Refractory Metals, by C. A. Krier inBattelleTechnical Review, June 1961, vol. 10, No. 6, pp. 11-15. Thisreference indicates that silicide coatings have been placed onmolybdenum bases by such methods as pack cementation. Uniformity ofcoating and adherence of the coating to the substrates have beenproblems in the prior art. A generally satisfactory method of applyingprotective diffusion coatings, including silicide, on metal bases isdisclosed in the copending application of the common assignee, S.N.85,457, filed January 30, 1961, in the name of Roger D. Moeller forDiffusion Coating Method for etals and Alloys. In this method aprotective coating is applied by placing the base metal in a moltenalkali metal bath having the coating material dissolved therein; thedissolved material diffuses through the bath on to the base metal. Theresulting articles have shown improved mechanical and chemicalproperties due to the formation of diffusion coatings, generallyinvolving intermctallic compound formation on the surface of the basemetal. Molybdenum silicide coatings prepared in such a manner, whileimproved over the prior art, showed, however, irregular thicknesses andvaried performance lifetimes. As u-scd herein, molybdenum refers both tothe metal and alloys thereof, for example Mo-Ti and Mc v Ti-Z-r.

lf id fid Patented June 2%, recs "ice In accordance with our presentinvention, we have developed an improved method of forming a molybdenumsilicide coating on molybdenum, which comprises providing a moltenalkali metal bath, dissolving therein silicon and at least one additiveselected from the class consisting of carbon and tin, and placing themolybdenum in the bath, thereby obtaining a coating of molybdenumsilicide on the molybdenum.

The molybdenum silicide coatings obtained in this mannor are found togreatly improve the oxidation resistance of molybdenum. The hardness,ductility. resistance to thermal shock, surface appearance, erosion andabrasion resistance, and resistance to mechanical impact are among theother properties which are improved. The term molybdenum silicide isused to identify the coating since the precise stoichiometry is notknown. It is probable that MoSi is the predominant phase, and such otherphases as 'MO3Si5 maybe present. Since molybdenum silicide itselfis'subject to oxidative attack, it is probable that the mechanism ofprotecting the molybdenum base involves the formation of SiO by reactionof oxygen and MoSi The Si0 is glassy and relatively impermeable tooxygen.

The additives improve appearance and performance as compared with amolybdenum silicide coating applied from an alkali metal bath.containing only silicon. The role of the additives are not fullyunderstood; however,

. it is postulated that any of the following mechanisms may be involved:the oxidation characteristics may be improved by formation ofintermediate molybdenum silicide phases; mechanical properties may beimproved by raising the ductility of the coating and making thecoefficient of thermal expansion more nearly that of the base metal; andthe kinetics of the coating process may be altered by retarding the backreaction consisting of dissolution of the coating in the bath, therebyresulting in a more even coating on the molybdenum.

Various examination methods, such as metallographic, X-ray diffraction,and spectographic, have not clearly shown the presence of carbides ortin compounds in the molybdenum silicide coatings. It would appearpossible, therefore, that the additives perform principally bycontrolling the bath conditions to produce a more uniform,

reproducible coating of high quality. The reaction rate with theadditives is slower than without the additives, and there is lessmolybdenum dissolution in the bath. For example, in the coating of aZS-mil molybdenum wire in sodium at a temperature of l4901500 F. underan inert gas atmosphere, a coating having an average, highly irregularthickness of 6 mils of molybdenum silicide is obtained after about /2hour and the original wire diameter is reduced to about 12 mils ofmolybdenum. With the additives, about 2-6 hours is required under thesame conditions to produce a molybdenum silicide coating of a uniform 3mils, but the original molybdenum wire diameter is reduced to only 23mils.

We find that the separate additions of carbon and tin individually serveto improve the quality of the coating, and that the combination of tinand carbon is particularly effective and is preferred. Carbon is usedherein to embrace both carbon and inorganic compounds thereof, includingsuch carbides as boron carbide; carbon in noncompounded form is thepreferred additive. The carbon additions serveto increase the thicknessof the coating and the oxidation performance of the coated metal;ductility is not significantly improved. The tin addition increases thethickness slightly, but uniformly, and improves the ductility andoxidation performance. The carbon and tin additions result in a veryattractive metal coating, having markedly improved oxidationperformance.

Our process may be suitably performed in any alkali metal bath. Sodiumis the preferred alkali metal medium,

and alloys thereof such as NaK are also very suitable. The process isoperable with even small silicon and additive concentrations, althoughthe coating rate may beretarded. It is generally preferred that thealkali metal bath contain saturation concentrations or" silicon and theadditives at the process operating temperature in order to improve thereaction rate. This will constitute, for example, in a sodium bathmaintained at a temperature of about 1500 F and containing 50-100 gramsof sodium,- silicon weighing approximately 10% of the weight of thesodium and added in the form of crushed crystals. Approximately 2 wt.percent of boron carbide or carbon may be added; the tin additions aresatisfactorily approximately 10 wt. percent.

The temperature of the alkali metal bath may satisfactorily vary over aconsiderablev range, for example 1000-2000 F., while it is found that atemperature of about 1400-1600 F., particularly for sodium, is optimum.Thebath is maintained under an inert gas atmosphere, for example argonor another noble gas. The time required for the coating process to becompleted varies with such parameters as the temperature of the bath andthe coating thickness desired. The additives slow down the molybdenumsilicide formation rate as compared with molybdenum silicide coatingfrom a similar hath not containing carbon or tin additives. The platingtime will vary between about /2-6 hours, depending on the coatingthickness desired; a time of about 3-5 hours is generally satisfactory.

The following examples are offered to illustrate our process in greaterdetail.

Example I The coating process was performed in a stainless steel capsule(type 304), 1.15-inch O.D., 0.05-inch wall thickness, -,-inch-thick endcaps, and 6.5 inches in length. The end caps were welded on, one in'theshop and the other in an argon atmosphere glove box after the capsulewas filled. The capsule was filled with 75 grams of sodium to about A ofits capacity, and a molybdenum wire 0.062 inch in diameter was placed inthe capsule. About 7.5 grams of silicon, 1.5 grams of carbon, and 5.0grams of tin were also placed in the capsule and the capsule thensealed. Two such samples were heated at 1450 .F. for 5 hours and a thirdsample at 1400 F. for 3 hours.

The coated wires were removed and subjected to selfresistance heating bypassing an electric current there-- Example 11 The procedure of ExampleI was followed except that 1.5 grams of carbon was the only additive tothe bath in addition to the silicon. The capsule was heated at 1400 F.for 3 hours. The coated wire was removed and subjected to resistanceheating at 1570 C. Time to failure was 38 hours-..

Example. 111

The procedure of Example I was followed except that 5.0 grams tin wasthe only additive to the silicon. The capsule was heated at 1450 F. for5 hours. The coated wire was removed and subjected to resistance,heating. The time to failure was 524 hours.

Example IV The same as Example I except that 1.5 grams of boron carbidewas the only additive in addition to the silicon.

. The capsule was heated at 1450 F. for /2 hour, and the sample removed.Two coated samples were subjected to resistance heating, and their livesto failure were 9 and 16 hours, at 1350 C.

Example V The same as Example I except that 5.0 grams of tin and 1.0gram of boron carbide were added to the bath. The capsule was heated at1450 F. for 5 hours. The coated sample. was removed, and subjected toresistance heating at 1570 C. until failure occurred after 54 hours.

Example VI The same as Example 1 except that the alkali metal bath was22% Nil-78% K, and the Mo wire was 0.025 inch in diameten The time tofailure of the test sample under resistance heatingin air at 3000 F. was22 hours.

Similar Wires coated in /2 hour at 1450 F. in a bath comprised of sodiumand silicon only yielded lifetimes of 1 hour at 1300 C., 0.2 hour at1350 C., and 0.7 and 9.0 hours at 1570 C.

The above examples are illustrative rather than restrictive of ourinvention, which should be understood to be limited only as is indicatedin the appended claims.

We claim:

- 1. A method of forming a molybdenum silicide coating of improveduniformity and adherence on molybdenum which comprises providing amolten alkali metal bath under an inert gas atmosphere, dissolvingeffective amounts of silicon and at least one additive selected from theclass consisting of tin and carbon in said bath, placing said molybdenumin said bath, and maintaining said molybdenum in said bath until saidcoating of molybdenum silicide is obtained.

2. The method of claim 1 wherein said bath is maintained at atemperature of approximately 1000-2000 F.

3. The method of claim 1 wherein said molybdenum is maintained in saidbath for a period of approximately /z6 hours.

4. The method of claim 1 wherein said molybdenum is maintained in saidbath at a temperature of approximately 1400-1600 F. for a period ofapproximately 35 hours.

5. A method of forming a molybdenum silicide coating on molybdenum,which comprises providing a molten alkali metal bath maintained at atemperature of approximately 1400-1600" F. under an inert gasatmosphere, dissolving therein approximately saturation concentrationsof silicon, carbon, and tin, placing said molybdenum in said bath, andmaintaining said molbydenum in said bath for a period of approximately3-5 hours, thereby forming a coating of molybdenum silicide on saidmolybdenum.

6 The method of claim 5 wherein said alkali metal is selected from theclass consisting of NaK and sodium.

7. The method of claim 5 where approximately 10 wt. percent silicon, 5wt. percent tin, and 1 wt. percent carbon are added to said alkali metalbath.

8. A method of forming a molybdenum silicide coating on molybdenum,which comprises providing a molten sodium bath maintained at atemperature of approximately 1400l600 F. under'an inert gas atmosphere,dissolving in said bath approximately 10 wt. percent silicon, 5 wt.percent tin, and 1 wt. percent carbon, placing said molybdenum in saidbath, maintaining said molybdenum in said bath for a period ofapproximately 3-5 hours, thereby forming a molybdenum silicide coatingon said molybdenum.

References Cited by the Examiner UNITED STATES PATENTS 2,848,352 8/58Noland et al 117-114 3,085,028 4/63 Logan l17ll4 3,086,886 4/63Kiefferet-al. 117-114 RICHARD D. NEVIUS, Primary Examiner.

1. A METHOD OF FORMING A MOLYBDENUM SILICIDE COATING OF IMPROVEDUNIFORMITY AND ADHERENCE ON MOLYBDENUM WHICH COMPRISES PROVIDING AMOLTEN ALKALI METAL BATH UNDER AN INERT GAS ATMOSPHERE, DISSOLVINGEFFECTIVE AMOUNTS OF SILICON AND AT LEAST ONE ADDITIVE SELECTED FROM THECLASS CONSISTING OF TIN AND CARBON IN SAID BATH, PLACING SAID MOLYBDENUMIN SAID BATH, AND MAINTAINING SAID MOLYBEDENUM IN SAID BATH UNTIL SAIDCOATING OF MOLYBDENUM SILICIDE IS OBTAINED.